Understanding SQL Query Execution Order: From FROM to LIMIT

In this article, a senior architect explains the logical execution order of a SQL query, illustrating each step with diagrams and code examples.

SQL Execution Order

The engine processes the query in the following sequence: FROM/JOIN to determine table relationships, WHERE for initial filtering, GROUP BY to group rows, HAVING for group‑level filtering (including aggregate functions), SELECT to compute final columns (adding aggregate results), DISTINCT to remove duplicates, ORDER BY to sort the result set, and finally LIMIT to restrict the number of rows returned.

FROM, JOIN, WHERE

These clauses define the tables involved and the join conditions. from table1 join table2 on table1.id=table2.id Or using a comma list with a WHERE condition:

from table1, table2 where table1.id=table2.id

GROUP BY

GROUP BY groups rows according to specified columns without filtering; the article shows an example of grouping by odd/even IDs.

HAVING and WHERE

HAVING can contain aggregate functions, while WHERE cannot. The article notes that using HAVING alone can replace WHERE for a smoother query.

100/2=50, splitting 100 into (10+10+...)/2 = 5+5+... = 50; the filtering condition remains unchanged, so grouping does not affect the result.

SELECT

After grouping, SELECT retrieves columns and aggregates, adding new fields for aggregate results.

select employee.id, distinct name, salary, avg(salary)

ORDER BY

ORDER BY sorts the final result set, for example by id.

LIMIT

LIMIT is applied last to truncate the result set after sorting. The article warns that applying LIMIT before ORDER BY can yield incorrect rows.

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